Method of an apparatus for generating ultra-short time-duration laser pulses

ABSTRACT

Both a method and an apparatus are disclosed for generating laser pulses having a time duration on the order of subnanoseconds. These light pulses also have a high output power. They are generated by causing a laser to lase and removing the bulk of the radiation contained in the laser cavity. The remaining radiation or light extends over only a short length of the cavity. This light pulse is mode locked, that is it consists of individual Fourier components of the proper phase so that the short light pulse retains its shape while being amplified again in the laser cavity. This short-duration light pulse may then be made to issue from the cavity. Alternatively, the amplified light may be made to issue as a pulse train or set of pulses. Finally, a single pulse may be utilized for mode locking another laser where the ultra-short time-duration pulse may be amplified again. This may be considered priming and subsequent mode locking of the second laser.

United States Patent Simmons [54] METHOD OF AN APPARATUS FOR GENERATINGULTRA-SHORT TIME- DURATION LASER PULSES [72] Inventor: William W.Simmons, Palos Verdes Peninsula, Calif.

[73] Assignee: TRW lnc Redondo Beach, Calif.

[22] Filed: May 14, 1971 [21] Appl. No.: 143,515

[52] US. Cl ..'...331/94.5, 350/160 [51] Int. Cl ..H01s 3/11 [58] Fieldof Search ..331/94.5; 350/160 [56] References Cited 1 UNITED STATESPATENTS 3,508,164 4/1970 Uchida ..331/94.5 3,521,069 7/1970 De Maria etal ..331/94.5 3,521,188 7/1970 Sooy ..33l/94.5 3,564,450 2/1971 lmmarcoet a1 ..331/94.5 3,564,454 2/1971 Hook et al. ..33l/94.5 3,571,7443/1971 Hook et a1 ..331/94.5 3,577,097 5/1971 Hilberg ..33l/94.5

Power Source 28 Primary ExaminerWilliam L. Sikes Attorney-Daniel T.Anderson, Edwin A. Oser and Jerry A. Dinardo ABSTRACT Both a method andan apparatus are disclosed for generating laser pulses having a timeduration on the order of subnanoseconds. These light pulses also have ahigh output power. They are generated by causing a laser to lase andremoving the bulk of the radiation contained in the laser cavity. Theremaining radiation or light extends over only a short length of thecavity. This light pulse is made locked, that is it consists ofindividual Fourier components of the proper phase so that the shortlight pulse retains its shape while being amplified again in the lasercavity. This short-duration light pulse may then be made to issue fromthe cavity. Alternatively, the amplified light may be made to issue as apulse train or set of pulses. Finally, a single pulse may be utilizedfor mode locking another laser where the ultra-short time-duration pulsemay be amplified again. This may be considered priming and subsequentmode locking of the second laser.

10 Claims, 7 Drawing Figures Voltage Generator 1 Pmilitiiwmnm I3.701.956

SHEET 1 BF 2 Voltage l6 Generator 54 II l0 I2 30 25 40 i P l5 ,7 l

umpmg 34 Power Source k- 22 24 48 :1 I Voltage i L Generatar 1 V0)Normal Polarization 1 48 Direction B g Signal v One-way Optical powerflow within cavity 0 I Time- 74 7 HQ 6 75 5 1 Voltage 64 K GeneratorWilliam W. Simmons INVENTOR.

Voltage aw/7O 5 I q Generator 73 BY ia Um ATTORNEY PAT-ENT'EDnmsusn01,955

SHEET 2 OF 2 38 v l I Kerr Cell, Pulse Retardation 4 ltl M Optical PowerAt Arrow 25 2L I V l r I I l (r-r) 1 Optical Power In Cavity I IncidentOn Mirror l6 53 I CAV I l I (r-t") Fig 3 Volt s v Optical Power Volts fa Fig-7 INVENTOR.

1 P o r lP w 34 p cal Power William W. Simmons" Time ATTORNEY METHOD OFAN APPARATUS FOR GENERATING ULTRA-SHORT TIME-DURATION LASER PULSESBACKGROUND OF THE INVENTION This invention relates generally to lasers,and particularly to a laser so operated as to generate a pulse having asubnanosecond time duration.

For many applications it is desirable to operate a laser in-such amanner that it generates output pulses of large energy having a timeduration on the order of nanoseconds or even less. However, in the pastit has been difficult, if not impossible, to generate a light pulsehaving a duration in the subnanosecond region which also has a highoutput power. 7

It is known to operate lasers by time-variable reflectivity. Such alaser has been disclosed and claimed in a U.S. Pat. No. 3,571,744 whichissued on Mar. 23, 1971 to Hook and Dishington and is entitled LaserIncorporating Time Variable Reflectivity. This patent is assigned to theassignee of the present application, The laser is initially made to lasein a conventional manner. Subsequently, by means of Qswitching all thelight contained in the laser cavity is reflected or refracted out of thelaser cavity. This may, for example, be effected by changing thepolarization of the light in such a manner that all the light containedin the cavity is switched out of the cavity by a birefringent prism orthe like. In that case, obviously the time duration of the light pulsecan be no less than the time duration of the light passing from one endof the cavity to the other and back again to' its origin, that is thetime it takes light to pass twice the length of the cavity.

Thus the time duration of such a light pulse may be in the order of afew nanoseconds and its amplitude or energy is relatively high. However,it is not possible to reduce the time duration of the light pulseobtained with such a time-variable reflectivity laser.

Another scheme for generating giant laser pulses of short time durationhas been disclosed in a patent to Witte and Frantz US. Pat. No.3,506,927 and entitled Selected Mode Giant Pulse Laser. This patent isalso assigned to the'assignee of the present invention. It is proposedhere to inject a radiation signal or pulse from a first laser into asecond laser thereby to amplify only oscillations of a desired modegroup. Accordingly the Witte and Franz giant pulse laser operates byinjection locking of the power laser.

It is accordingly an object of the present invention to provide a methodof and apparatus for generating laser pulses having a time duration onthe order of subnanoseconds.

Another object of the present invention is to provide a practical methodof injection locking of a laser for the purpose of generatingultra-short time-duration light pulses of high power. 7

A further object of the present invention is to generate an amplifiedlight pulse in the laser cavity which is short compared to the length ofthe laser cavity by mode locking of the laser.

SUMMARY OF THE INVENTION In accordance with the present invention amethod is provided of operating a laser to generate a pulse of radiationhaving an ultra-short time duration. To this end the laser which has anoptical cavity is caused to lase until the cavity is substantiallyfilled with radiation.

Thereafter the bulk of the radiation contained in the cavity is removed.As a result radiation remaining in the cavity extends only over a shortlength of the cavity. The radiation remaining in the cavity is nowamplified by laser action to develop an amplified, short-duration pulse.This is made possible because the pulse has Fourier components whichmatch the Fabry-Perot resonances of the cavity. Thus the short timepulse may be considered to consist of radiation components extendingover a certain frequency range and having such phase relationships thatthe laser is mode locked. Therefore, the pulse remains together while itis reflected back and forth between the mirrors defining the cavity ofthe laser and while the pulse is being amplitied. Finally, the shortpulse which is at a high power level is utilized.

This may be effected by causing the short-duration pulse to issue fromthe cavity. Alternatively it is feasible to issue a pulse trainconsisting of a series of amplified pulses. Finally the short durationpulse may be utilized for triggering or priming another laser, that is,for injec tion locking the second laser. The second laser may bearranged to have high amplification to produce an output pulse with thesame short duration but having even a higher energy level.

The novel features that are considered characteristic of this inventionare, set forth with particularlity in the appended claims. The inventionitself, however, both as to its'organ'ization and method of operation,as well as additional objects and advantages thereof, will best beunderstood from the following description when read in connection withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS I FIG. 1 is a schematicrepresentation, partly in block form, of a laser system in accordancewith the present.

invention for generating an ultra-short time-duration 4 generating veryshort time duration pulses which may be used with the laser of FIG. 1;

FIG. 5 is a chart illustrating the optical power as a function of timewithin the cavity of a laser when the laser is operated in a particularway to change the relative mode phasing of the light remaining in thecavity;

FIG. 6 is a schematic representation, partly in block form, of amodified laser system in accordance with the present invention forinjecting a light pulse generated in a first laser into a secondlaserfor further amplification of the mode-locked pulse; and

FIG. 7 is a set of curves plotted as a function of time to illustratethe voltages applied to various Kerr cells of the system of FIG. 6 aswell as the optical power remaining in the two laser cavities of thesystem of FIG. 6.

Referring now to the drawings, and particularly to FIG. 1, there isillustrated a laser system for generating high-power, ultra-shorttime-duration pulses in a accordance with the present invention. Thelaser system of FIG. 1 includes a laser generally indicated at 10. Thelaser may, for example, be a gaseous laser as shown and may .be providedwith a pair of windows 11 and 12 arranged at the Brewster angle tominimize reflection of the laser light. Also the light passing throughthe Brewster windows 11 or 12 will be linearly polarized. However, itshould be understood that any conventional laser may be substituted forthe gaseous laser such, for example, as a solid state laser. The laseris provided with a pumping power source 14 which may be connected forexample in the case of a gaseous laser to a pair of electrodes 15immersed in the gas. However, it will be understood that in the case ofa solid state laser, the pumping power source 14 may energize, forexample, a flash lamp for optical pumping of the laser.

The optical cavity is defined by a pair of mirrors l6 and 17. Themirrors may, for example, be spherical mirrors as shown whichfacilitates adjustment of the cavity. However, it is feasible instead touse mirrors with plane surfaces. Preferably the two mirrors 16 and 17are coated at their opposed surfaces. This may, for example, be effectedby a suitable dielectric coating. In general, the mirrors l6 and 17should be made completely reflecting. In practice they may have areflectivity of about 99 percent.

Further disposed in the optical cavity is an electrooptical element 18which may, for example, consist ofa Kerr cell as shown or alternativelyof a Pockels cell or of an acoustic cell. In general any electro-opticaldevice may be used which is capable of changing the polarization oflight passing therethrough in response to an applied voltage or the likesignal. Thus the electro-optical element 18 such as a Kerr cell may beprovided with two electrodes 20 and 21 one of which is grounded whilethe other one is connected to a voltage generator 22. Thevoltagedeveloped by the generator 22 will be discussed hereinafter inconnection with FIG. 3.

Further disposed in the optical cavity is a birefringent element 24which may, for example, consist of a calcite prism as shown. However,any birefringent element may be substituted therefor. It has the purposeof refracting the light passing therethrough selectively either betweenmirrors 16 and 17 or in the direction of arrows 25 or 26 depending onthe polarization of the light passing therethrough. To this end it mustbe assumed that the light reflected between mirrors l6 and 17 has anormal direction of polarization. If the light issued by the laser 10 isnot normally so polarized this maybe effected by a suitable polarizerinterposed, for example, between laser 10 and Kerr cell 18. However, thelight passing a laser having windows 11 and 12 disposed at the Brewsterangle will automatically be linearly polarized.

It will be noted that the calcite prism 24 refracts the light so that acavity is used consisting of two branches disposed substantially atright angles to each other with the prism 24 at the intersection of thetwo cavities.

However, this particular arrangement of the optical cavity is notessential for the operation of a laser system in accordance with thepresent invention. Thus the laser of FIG. 6 which will be discussedlater on consists of two optical cavities each being disposed along asingle line and intersecting each other at right angles.

The laser system of FIG. 1 may optionally be provided with a secondelectro-optical element 27 which may be identical with the element 18and may consist of a Kerr cell as shown. It also has a pair ofelectrodes 28 and 30 one of which is grounded as shown while the otherone is connected to a voltage generator 31 which may be similar to thevoltage generator 22.

Thus with the normal direction of polarization of the light, the lightwill move in the direction shown by the dotted line 32 through the prism24 as shown by line 33 and will be refracted out of the prism as shownby the line 34. The laser is permitted to lase by energizing it throughthe pumping power source 14. This is continued until the entire lasingcavity is fllledwith light radiation reflected back and forth betweenmirrors l6 and 17. Thus the light power builds up to some steadystateoptical radiation level. This may be denoted by the lasing energy W andit may be assumed that this energy is uniformly distributed throughoutthe cavity. The cavity' may have a length L so that the optical powerflow in one direction is as follows: Wc/2L, where c is the velocity oflight.

2 After this steady state has been reached a voltage pulse V(t) isapplied by the voltage generator 22 to the Kerr cell 18.-- This voltagepulse should have a very fast rise time. This is more clearly shown inFIG. 2 which may be considered to be an end view of the Kerr cell 18 sothat the light passes through the cell 18 in a direction at right anglesto the paper plane. The normal polarization direction is shown in FIG. 2by the arrow. Accordingly. the voltage V(t) creates a new polarizationcomponent as shown by the dotted arrow 36. This polarization componentis in the direction of the horizontal plane. It will be assumed for thefollowing discussion that the light passes twice through the Kerr cell18. To this end the Kerr cell 18 may be disposed close to one of themirrors such as 17. A fractional component of the light in thehorizontal direction may be represented as follows: A sin(d /2), where4: is the phase delay introduced by the voltage applied to the Kerr cell18. This phase delay or retardation of the light due to the Kerr cell isshown in FIG. 3 at 38. As shown here it preferably has a steep rise andfall time and is substantially flat topped. It also has a very shorttime duration as will be more fully explained hereinafter.

Due to the application of the voltage pulse to the Kerr cell the lightwave passing through the Kerr cell 18 is retarded as shown at 38 in FIG.3. This in turn will change the polarization of the light so that thelight passing through the prism '24 is' refracted as shown by the dottedline 40 and emerges as shown in the direction of arrow 25.

If all the light contained in the optical cavity is removed by the pulseapplied to the Kerr cell 18 the operation is like that of a conventionaltime variable reflectivity laser as disclosed in the previously referredto patent to Hook and Dishington. However, in ac-' cordance with thepresent invention not all of the radiant energy contained in the cavityis removed or dumped. Thus the electrical pulse applied to the Kerr cell18 should be sufficiently short so that radiant energy in a localizedportion of the cavity remains in the cavity. What is'desired is that theremaining light in the cavity is localized in the form of an opticalpulse. It occupies a region in the cavity having a length of c(-r 1-)I.., where 1' is the rise or fall time of the electrical pulse V(t).Further 'r r is the-time duration of the light pulse. 1- 'r' E ZL/c l/N,where N is the total number of modes of the light pulse.

This remaining light pulse now passes back and forth by reflectionbetween the mirrors l6 and 17. During this time, of course, the laser iscontinuously pumped by the pumping power source 14. As a result thecirculating pulse is amplified'every time it passes through the laser10. After a number of passes of the pulse it has again built up to itsmaximum power level as the laser approaches again its steady-stateoperating condition. It may now be assumed that the laser has againarrived at an energy content W and the peak power of the optical pulseis given by the following relationship Essentially the energy of theamplified pulse is substantially identical with the energy of all theradiation initially contained in the laser cavity.

Referring again to FIG. 3, curve 42 illustrates the optical power at thearrow which is removed from the amplitudes increase much beyond theamplitude curve of 43 until they reach the level of the relationship(1). The light pulses follow each other at time intervals of 2L/c.

Relationship (1) is strictly true only if the laser functions as adistortion-free amplifier while the amplitude of the pulses increase asshown in 44. Actual experiments have shown that the optical pulsespreads somewhat as the laser approaches saturation or its steady-statecondition.

It should be noted that the time duration of pulse 42 is on the order of3 to 4 nanoseconds (nsec). This is the usual limit of an output pulseobtained from a time variable reflectivity laser. On the other hand thetime duration corresponding to 1' 1" is on the order of 0.3 nsec for anargon ion laser which is at least an order of magnitude shorter induration.

In general the peak power P, of the pulse remaining in the cavity isgiven by the following formula:

W6 1r 'r'r' 4 52 (5) and the energy content AW of the pulse is given asfollows:

' W 1r 4 c-r 'r' 5 5) (21) e) It should be noted that AW should belarger than the spontaneous emission energy of the laser during thepulse duration'time. This, of course, is the energy of radiation createdby a spontaneous emission in the laser.

FIG. 4 to which reference is now made showsan example of a voltagegenerator which may be used for generating an electrical pulse of therequired time duration which may be on the order of a few nsec. Thecircuit of FIG. 4 includes a gas tilled tube such, for example, as athyratron 46 having its cathode grounded while it control grid isconnected to a trigger signal source 47. The anode may beconnected to apositive voltage supply +B through a resistor 48. 'Further connected tothe anode of the thyratron 46 may be a coaxial transmission line 50having its far end short-circuited and grounded. Such a transmissionline will reflect any pulse applied to it within a time perioddetermined by the velocity of the pulse through the transmission line,and its length. Thus it is possible to develop a pulse having a veryshort duration by the use of a short transmission line. The outputterminals 51 may be connected respectively to the anode of thyratron 46and ground.

As indicated before the short light pulse remaining in accordance withthe present invention in the laser cavity consists of many oscillatinglaser modes of the proper phase with respect to each other. Thus thepulse may have a frequency spectrum extending over a range of a fewgigacycles. This pulse 'may be considered to have Fourier componentswhich match the F abry-Pe rot resonances of the optical cavity formed bythe two mirrors l6 and 17. t

The amplified pulse such as pulse 53 created after sufficientamplification in the optical cavity may then be made to issue from thelaser by now energizing the Kerr cell 27 by applying thereto a shorttime pulse with the voltage generator 31. This will change thepolarization of the light in such a manner that the light entering theprism 24 as shown by the dotted line 32.is refracted as shown by thedotted line 54 and emerges in the direction of arrow 26 where it may beutilized.

Alternatively it is feasible to make one of the mirrors 16 or 17 onlypartially reflective so that it may have a reflectivity say of 50 to 60percent. In that case the light may be made to issue say from the mirror17. In that case instead of obtaining a single light pulse 53 a train oflight pulses is obtained somewhat like the set of pulses shown' in FIG.3 at 44. This train of optical pulses may have a fractional nanosecondduration for each pulse and a repetition period of 2L/c which may be onthe order of Mc.

' Instead of applying one pulse to the Kerr cell 18 by means of voltagegenerator 22 to initiate the amplification of the short pulses and asecond pulse to the Kerr cell 27 by means of the voltage generator 31 toissue the amplified pulse, it is also feasible to utilize only a singleelectro-opticalcellsuch as a cell 18. In this case two separate voltagepulses must be applied to the same ,electro-optical cell, the-two pulsesbeing separated and timed properly to permit the short time-durationpulse to be amplified sufficiently.

While it may be difficult to operate a single Kerr cell with therequired double electric pulse by means of thyratrons it is feasible toobtain such an operation, for example, with a Pockels cell and improvedelectronic components such as a KN-6 Krytron tube instead of athyratron.

So far the discussion concerned the localization of the optical energyin as small a space as possible within the optical cavity. This, ofcourse, will achieve maximum output pulse power. However, variation ofthe applied voltage to the Kerr cell 18 changes the relative modephasing of the light remaining in the cavity. This in turn dramaticallyaffects the envelope of the laser recovery. Thus a curve such as shownin 55 in FIG. has been obtained. This depicts the one way optical powerflow within the cavity as a function of time. The

envelope is in the form of an electronic stairstep wave and risesaccording to an approximate exponential. The time duration of each stepis again 2L/c. A curve such as shown in 55 in FIG. 5 may be obtained ifenough light is left within the cavity to fill substantially the entirecavity.

The ultra-short time-duration laser pulse generated in accordance withthe present invention may also be used for injection locking a secondpower laser. Thus a first laser may be utilized to generate the pulsewhile a second laser with a high amplification may be utilized tofurther amplify the pulse. This is generally similar to mode locking ofa laser. The high amplification laser is primed by a short pulse. Thiscan be accomplished with the laser system shown in FIG. 6 to whichreference is now made.

The laser system of FIG. 6 consists essentially of two lasers havingtheir cavities disposed at right angles to each other.

The first laser 60 is enclosed by pass the pair of mirrors 61 and 62which together form an optical cavity extending along a straight line. Afirst Kerr cell 63 designated K, and a second Kerr cell 64 designated Kare disposed in the optical cavity formed by the two mirrors 61 and 62.A birefringent double prism 65 such as a Glan-Thompson polarizer formsthe intersection between the first and the second laser cavity. Thepurpose of the double prism 65 is to either the light straight throughthe prism or to reflect it at right angles depending on the state ofpolarization of the light passing therethrough.

The second laser system includes a laser 70 disposed in an opticalcavity formed by mirrors 71 and 72. Again two Kerr cells 73 and 74 aredisposed in the optical cavity and are designated respectively K, andFinally another birefringent double prism or Glan- Thompson polarizer 75is provided in the second laser cavity to permit light to be ejected outof the cavity.

The first laser defined by mirror 61 and 62 may be operated in themanner previously disclosed. Thus the light is initially polarized sayby the Brewster windows of the laser 60 in such a manner that the lightpasses straight through the prism 65. The first Kerr cell K is initiallyenergized by a voltage generator 76 to develop a voltage pulse as shownat 77 in FIG. 7. This will change the polarization of the light in sucha manner that the bulk of the light is ejected out of the cavity 61, 62by the prism 65 into the cavity 71, 72. However, at that time the secondlaser 70 is not being pumped so that this light is not being amplified.Furthermore, the second cavity 71, 72 has a low Q so that its light isejected out of the cavity by prism 75 as shown by arrow 87.

As a result, a short pulse remains in the cavity 61, 62. This amplifiedpulse train such as shown in 78 in FIG. 7

is generated in the first cavity 61, 62 and is designated as Pm.

A sufficiently amplified light pulse is now removed from the first lasercavity 61, 62 by applying a step voltage 80 to the Kerr cell K as shownin FIG. 7. This voltage may be applied by the voltage generator 81connected to the Kerr cell K;,. This will now change the polarization ofthe light in such a manner that the light is reflected at right anglesby prism 65 and injected into thelaser cavity 71, 72.

At the same time the laser cavity 71, 72 is Q- switched by the voltages83 and 84 applied respectively to the Kerr cells K, and K These voltagesmay be generated by the voltage generators 81 and 76 connected to therespective Kerr cells. The voltage 84 applied to the Kerr cell K changesthe polarization of the light to permit it to pass through prism 75between mirrors 71, 72. The voltage pulse 83 changes the polarization ofinjected light to permit it to pass through prism 65 between mirrors 71,72. The laser has previously been pumped to a high inversion of itspopulation just prior to the time T, that is prior to the time of thevoltage pulses 83 and 84.

Accordingly the injected laser pulse is amplified in the optical cavity71, 72 as shown by the pulse train 85 of FIG. 7 designated P The opticaloutput power is now obtained by removing the voltage that was previouslyapplied to the Kerr cell K, as shown by the voltage wave 84. This willnow change the polarization of the light so that the light is reflectedby the. prism in the direction of arrow 87. The resulting output pulse88 is also shown in FIG. At this time the'voltage wave may again beremoved from the Kerr cell K,. It will be noted that a low-amplitudeoptical pulse 90 is also shown in FIG. 7 as the optical power output.This corresponds to the light energy which was initially dumped from thelaser cavity 61, 62 into the laser cavity 71, 72 and which appears inthe direction of output arrow 87.

There has thus been disclosed a method of and apparatus for generatinglaser pulses having an ultra-short time duration. In spite of theirshort duration, on the order of less than 1 nanosecond, the pulses canbe made to have a substantial output power. It is feasible either togenerate a single pulse or to provide a pulse train of lesser power.Alternatively it is feasible to provide optical pulses which increase inamplitude in the manner of a stairstep wave and which have a time duration each corresponding to the time it takes light to traverse theoptical cavity twice. Finally the ultra-short duration output pulses maybe used for injection locking a second power laser which in turn mayfurther amplify the original pulse. The advantage of the laser system ofthe invention is that the output pulses have a time duration of lessthan a nanosecond while still having a power or energy corresponding tothat of the pulses obtained, for example, by time variable reflectivity.They may also be used for injection mode locking which is generallyrather difficult to accomplish.

What is claimed is: I

1. The method of operating a laser to generate a pulse of radiationhaving an ultra-short time duration, said method comprising the stepsof:

a. causing a laser having an optical cavity to lase,

thereby substantially filling the cavity with radiation;

b. removing radiation contained in the cavity for a predetermined timeperiod less than 2 L/c, where L is thelength of the optical cavity and cthe velocity of the radiation so that radiation remains in the cavityextending only over a short length of the cavity;

c. continuing to amplify the radiation remaining in the cavity todevelop an amplified short-duration pulse; and

d. removing the amplified pulse from the laser cavity.

2. The method as defined in claim 1 wherein a portion of each of aseries of amplified pulses is removed from the laser, each pulse havinga duration short compared to 2 U0.

3. The method defined in claim 1 wherein the removed amplified pulse isinjected into another laser for injection mode-locking and furtheramplification.

4. The method of' operating a laser to generate a stairstep waveform ofradiation, said method comprising the steps of: v

a. causing a laser having an optical cavity to lase,

thereby substantially tofill the cavity with radianon;

b. causing the radiation to have a first predetermined direction ofpolarization;

c. causing radiation having a second predetermined direction ofradiation to be removed from the cavid. removing a portion of theradiation contained in the cavity by rotating the first direction of 1polarization of the radiation into a third direction of polarization fora predetermined period of time less than 2 L/c, where L is the length ofthe optical cavity and c is the velocity of the radiation, the thirddirection of polarization being between the first and the seconddirection of polarization whereby a portion, but not all of theradiation, is removed from the cavity during the predetermined period oftime, whereby a low energy level remains in a portion of the length ofthe cavity said substantially all of the energy of the radiation remainsin the cavity for a period of time of 2 L/c less the predeterminedperiod of time;

e. amplifying the radiation remaining in the cavity and having a powerof the shape of a stairway flowing within the cavity; and

f. removing the amplified radiation from the cavity.

5. Apparatus for generating an ultra-short time-duration pulse, saidapparatus comprising: I

a. an optical cavity defined by a pair of substantially completelyreflecting mirrors;

b. a laser disposed in said cavity;

. c. means for pumping said laser to cause it to lase;

d. an electro-optical element disposed in said cavity, said elementbeing capable of changing the polarization of light passing therethroughin response to an applied voltage;

e. a birefringent element disposed in said cavity, said birefringentelement being normally arranged for passing light having a firstpredetermined polarization from one of said mirrors to the other andbeing capable of refracting light having a second predeterminedpolarization out of said cavity;

f. a signal generator coupled to said electro-optical element forapplying thereto a signal having a predetermined time duration of lessthan 2 L/c,

where L is the distance between said mirrors and c is the velocity oflight, whereby the polarization of the light passing through said laseris changed from said first to said second predetermined polarization soas to cause said birefringent element to reflect said light. out of saidcavity during said predetermined time duration; and

g. continuing to maintain said means for pumping energized, thereby toamplify the light pulse remaining in said cavity having a length shortcom-' pared to that of said cavity. 6. Apparatus for generating anultra-short time-duration pulse, said apparatus comprising:

a. an optical cavity defined by a pair of substantially completelyreflecting mirrors, said mirrors being so positioned as to form twobranches forming an angle with each other;

b. a laser disposed in one of said branches;

0. means for pumping said laser to cause it to lase;

d. an electro-optical element disposed in one of said branches, saidelement being capable of changing the polarization of lightpassingtherethrough in response to an applied voltage;

e. a birefringent prism disposed at the intersection of said branches ofsaid cavity, said prism being normally arranged for refracting lighthaving a first predetermined polarization from one of said branches tothe other and being capable of refracting light having a secondpredetermined polarization out of said cavity;

f. a voltage generator coupled to said element for applying thereto apulse having a predetermined time duration of less than 2 L/c, where Lis the length of said cavity and c is the velocity of light, whereby thepolarization of the light passing through said element is changed fromsaid first to said second predetermined polarization so as to cause saidprism to reflect said light out of said cavity for said predeterminedtime duration; and g. continuing to maintain said means for pumpingenergized thereby to amplify the light remaining within said cavity andhaving a length short compared to that of said cavity.

7. Apparatus as defined in claim 6 wherein said voltage generatordevelops a second short time-duration pulse for permitting the amplifiedradiation in said cavi ty to issue from said cavity.

8. Apparatus as defined in claim 6 wherein said electro-optical elementconsists of two Kerr cells, each being disposed in one of said branchesand wherein said voltage generator is coupled to one of said Kerr cells,and an additional voltage generator coupled to the other one of saidKerr cells, said additional voltage generator developing av second shorttime-duration pulse for causing the amplifiedradiation in said cavity toissue out of said cavity.

9. Apparatus as defined in claim 6 wherein one of said mirrors ispartially transparent, thereby to issue a train of amplified pulsesafter said light has been refracted out of said cavity.

10. Apparatus as defined in claim 6 wherein said pulse of predeterminedduration has such a voltage that the polarization of the light passingthrough-said element is changed to a value between that of said firstand said second predetermined polarization, whereby a

1. The method of operating a laser to generate a pulse of radiationhaving an ultra-short time duration, said method comprising the stepsof: a. causing a laser having an optical cavity to lase, therebysubstantially filling the cavity with radiation; b. removing radiationcontained in the cavity for a predetermined time period less than 2 L/c,where L is the length of the optical cavity and c the velocity of theradiation so that radiation remains in the cavity extending only over ashort length of the cavity; c. continuing to amplify the radiationremaining in the cavity to develop an amplified short-duration pulse;and d. removing the amplified pulse from the laser cavity.
 2. The methodas defined in claim 1 wherein a portion of each of a series of amplifiedpulses is removed from the laser, each pulse having a duration shortcompared to 2 L/c.
 3. The method defined in claim 1 wherein the removedamplified pulse is injected into another laser for injectionmode-locking and further amplification.
 4. The method of operating alaser to generate a stairstep waveform of radiation, said methodcomprising the steps of: a. causing a laser having an optical cavity tolase, thereby substantially to fill the cavity with radiation; b.causing the radiation to have a first predetermined direction ofpolarization; c. causing radiation having a second predetermineddirection of radiation to be removed from the cavity; d. removing aportion of the radiation contained in the cavity by rotating the firstdirection of polarization of the radiation into a third direction ofpolarization for a predetermined period of time less than 2 L/c, where Lis the length of the optical cavity and c is the velocity of theradiation, the third direction of polarization being between the firstand the second direction of polarization whereby a portion, but not allof the radiation, is removed from the cavity during the predeterminedperiod of time, whereby a low energy level remains in a portion of thelength of the cavity and substantially all of the energy of theradiation remains in the cavity for a period of time of 2 L/c less thepredetermined period of time; e. amplifying the radiation remaining inthe cavity and having a power of the shape of a stairway flowing withinthe cavity; and f. removing the amplified radiation from the cavity. 5.Apparatus for generating an ultra-short time-duration pulse, saidapparatus comprising: a. an optical cavity defined by a pair ofsubstantially completely reflecting mirrors; b. a laser disposed in saidcavity; c. means for pumping said laser to cause it to lase; d. anelectro-optical element disposed in said cavity, said element beingcapable of changing the polarization of light passing therethrough inresponse to an applied voltage; e. a birefringent element disposed insaid cavity, said birefringent element being normally arranged forpassing light having a first predetermined polarization from one of saidmirrors to the other and being capable of refracting light having asecond predetermined polarization out of said cavity; f. a signalgenerator coupled to said electro-optical element for applying thereto asignal having a predetermined time duration of less than 2 L/c, where Lis the distance between said mirrors and c is the velocity of light,whereby the polarization of the light passing through said laser ischanged from said first to said second predetermined polarization so asto cause said birefringent element to reflect said light out of saidcAvity during said predetermined time duration; and g. continuing tomaintain said means for pumping energized, thereby to amplify the lightpulse remaining in said cavity having a length short compared to that ofsaid cavity.
 6. Apparatus for generating an ultra-short time-durationpulse, said apparatus comprising: a. an optical cavity defined by a pairof substantially completely reflecting mirrors, said mirrors being sopositioned as to form two branches forming an angle with each other; b.a laser disposed in one of said branches; c. means for pumping saidlaser to cause it to lase; d. an electro-optical element disposed in oneof said branches, said element being capable of changing thepolarization of light passing therethrough in response to an appliedvoltage; e. a birefringent prism disposed at the intersection of saidbranches of said cavity, said prism being normally arranged forrefracting light having a first predetermined polarization from one ofsaid branches to the other and being capable of refracting light havinga second predetermined polarization out of said cavity; f. a voltagegenerator coupled to said element for applying thereto a pulse having apredetermined time duration of less than 2 L/c, where L is the length ofsaid cavity and c is the velocity of light, whereby the polarization ofthe light passing through said element is changed from said first tosaid second predetermined polarization so as to cause said prism toreflect said light out of said cavity for said predetermined timeduration; and g. continuing to maintain said means for pumping energizedthereby to amplify the light remaining within said cavity and having alength short compared to that of said cavity.
 7. Apparatus as defined inclaim 6 wherein said voltage generator develops a second shorttime-duration pulse for permitting the amplified radiation in saidcavity to issue from said cavity.
 8. Apparatus as defined in claim 6wherein said electro-optical element consists of two Kerr cells, eachbeing disposed in one of said branches and wherein said voltagegenerator is coupled to one of said Kerr cells, and an additionalvoltage generator coupled to the other one of said Kerr cells, saidadditional voltage generator developing a second short time-durationpulse for causing the amplified radiation in said cavity to issue out ofsaid cavity.
 9. Apparatus as defined in claim 6 wherein one of saidmirrors is partially transparent, thereby to issue a train of amplifiedpulses after said light has been refracted out of said cavity. 10.Apparatus as defined in claim 6 wherein said pulse of predeterminedduration has such a voltage that the polarization of the light passingthrough said element is changed to a value between that of said firstand said second predetermined polarization, whereby a portion of all ofthe light in said cavity remains in said laser while all of the lightfor said predetermined time duration remains in said cavity to form amode-phased pulse capable of generating an amplified wave within saidcavity of generally stairstep power shape.